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When your part needs grinding and what that does to the quote

August 25, 2026

Grinding is what you reach for when milling and turning run out of precision. It uses an abrasive wheel to remove very small amounts of material, which is why it holds tolerances and finishes the other processes cannot, and why it is slow enough that nobody uses it to remove bulk material.

The three types you will meet

Surface grinding produces flat faces and parallel surfaces. The part sits on a magnetic chuck and the wheel passes over it. This is the answer for a plate that has to be flat and parallel within a few microns, or for a hardened face that has to be true after heat treat.

Cylindrical grinding produces round diameters to close tolerance and fine finish. The part turns between centres or in a chuck. Use it for bearing seats, shaft journals, sealing surfaces and anything running against a seal.

Centreless grinding feeds the part between a grinding wheel and a regulating wheel without holding it in a chuck. It is the volume process for pins, dowels and shafts where the whole outside diameter has to be one size. On quantities in the thousands it is dramatically cheaper than cylindrical grinding.

What grinding actually buys you

Attribute	Typical from milling or turning	Typical after grinding
Dimensional tolerance	±0.02 mm	±0.005 mm
Flatness over 100 mm	0.05 mm	0.005 to 0.01 mm
Surface roughness	Ra 1.6 to 3.2	Ra 0.2 to 0.8
Roundness	0.01 mm	0.002 mm

Those are figures shops hold routinely on the right machine, not promises for your part. Geometry, material and size all move them.

Design decisions that make grinding cheaper or impossible

Leave grinding stock. If a surface is to be ground, the part is machined slightly oversize and the grinder removes the last 0.1 to 0.3 mm. Your drawing does not need to show this, but your process does, so say which surfaces are ground so the shop plans the stock.

Provide wheel runout. A ground diameter that runs into a shoulder needs an undercut for the wheel to exit. Without it the corner cannot be ground cleanly. This is the single most common drawing issue on ground parts.

Watch for interrupted surfaces. A diameter with a keyway or a cross hole through it can still be ground, but the wheel takes a hit at each interruption and the finish suffers.

Think about heat. Grinding puts heat into the surface. On hardened steel, aggressive grinding can cause burn and microcracks. If your part is fatigue critical, say so and ask whether a finer wheel and slower feed should be used, or whether a post-grind inspection is needed.

How to specify it

You do not have to write “grind” on the drawing. You have to write a tolerance and a finish that only grinding can meet, then let the shop decide. What helps a shop quote accurately:

• Which surfaces carry the tight tolerance, marked clearly, not a blanket note
• Hardness and heat treat state, because a ground surface on a hardened part is a different operation from a ground surface on annealed stock
• Surface finish as an Ra value, with the direction of lay if it matters for a seal
• Whether the ground surfaces relate to each other, for example parallelism between two ground faces, which decides whether they are ground in one setup

The cost conversation worth having

Grinding is an extra operation, an extra setup and sometimes an extra supplier. On a low quantity order it can be a third of the part price. Two questions usually save money:

1. Does every tight surface need to be tight? Very often one of the three toleranced features is doing the work and the others inherited a tolerance from a template.
2. Could a different fit do the job? A ground press fit can sometimes become a machined slip fit with a retaining compound, and the cost difference is large.

If the answer to both is no, the part needs grinding and that is fine. The important thing is that the request says so, because it changes the shop list. Not every machine shop in Vietnam has a surface grinder on the floor, and the ones that do usually came out of the tooling sector.

Alternatives worth checking first

Grinding is a real operation with a real price, so it is worth asking whether the requirement can be met another way.

Hard turning. Modern inserts turn hardened steel up to around 60 HRC and can reach finishes close to ground quality on round parts. Fewer shops offer it, and where they do it can replace a cylindrical grinding step entirely.

Reaming or boring instead of internal grinding. A reamed hole holds ±0.01 mm comfortably. If the requirement is size rather than form, this is usually enough.

Lapping or honing. For bores that need both size and surface, honing beats grinding on finish and is common in hydraulic work.

A better fit rather than a tighter tolerance. A press fit that needs grinding may become a slip fit plus a retaining compound. The assembly behaves the same and the part costs less.

What the shop needs to quote it properly

• Which surfaces are ground, and what they relate to
• Hardness and heat treat condition at the time of grinding
• Surface finish as an Ra value, with lay direction if a seal runs on it
• Whether wheel relief undercuts may be added where they are missing
• Whether the ground features are checked on a CMM or with hand instruments

The last point matters more than buyers expect. A part ground to ±0.005 mm and checked with a micrometer in a hot workshop has not really been checked.

The guide on tolerances covers how to mark up a drawing so this decision is obvious to whoever reads it, and you can send your drawing here when you want quotes from shops that hold this kind of work.

Questions we get on this

Keep reading

• 3-axis, 4-axis or 5-axis: which one your part actually needs
• Milling, turning or both: reading your part before you ask for a quote